Add dependabot (#2826) #2529

Job	Run time
cargo test (ubuntu-latest)	3m 13s
cargo bench	44s
cargo test (macos-latest)	5m 26s
cargo +stable build	36s
cargo clippy	25s
cargo +beta build	42s
cargo test (windows-latest)	4m 25s
cargo +1.36 build (futures-{core, io, sink})	58s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 37s
cargo +nightly build	43s
cargo doc	19s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 38s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 39s
cargo fmt	3s
cargo hack check --feature-powerset	2m 28s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 21s
cargo miri test	8m 17s
cargo build --target thumbv7m-none-eabi	27s
cargo build --target thumbv6m-none-eabi	29s
cargo test -Z sanitizer=address	4m 50s
cargo minimal-versions build	36s
cargo test -Z sanitizer=memory	4m 1s
cargo test -Z sanitizer=thread	2m 45s
cargo +1.56 build	1m 33s
	1h 4m 15s

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